meyer lemon tree white powder under some leaves also ants.

Question

what is the problem what can I do?

Answer 1

It sounds like you have powdery mildew. Your tree needs a couple of treatments with neem oil. Here's a link for an article about how to treat powdery mildew: https://www.gardeningknowhow.com/plant-problems/disease/powdery-mildew-homemade-and-organic-remedies.htm

It also could be insects, such as mealy bugs, but the Neem oil should take care of that, too.